When Comets Attack: Solving the Mystery of the Biggest Natural Explosion in Modern History

On the morning of June 30, 1908, the sky exploded over a remote region of central Siberia. A fireball as powerful as hundreds of Hiroshima atomic blasts scorched through the upper atmosphere, burning nearly 800 square miles of land. Scientists today think a small fragment of a comet or asteroid caused the "Tunguska event," so named for the Tunguska river nearby. Now, a controversial new scientific study suggests that a chunk of a comet caused the 5-10 megaton fireball, bouncing off the atmosphere and back into orbit around the sun. The scientists have even identified a candidate Tunguska object—now more than 100 million miles away—that will pass close to Earth again in 2045. Is there a hidden, but powerful, danger inside the seemingly harmless comet?

On the morning of June 30, 1908, the sky exploded over a remote region of central Siberia. A fireball as powerful as hundreds of Hiroshima atomic blasts scorched through the upper atmosphere "as if there was a second sun," according to one eyewitness. Scientists today think a small fragment of a comet or asteroid caused the "Tunguska event," so named for the Tunguska river nearby. No one knows for certain, however, because no fragment of the meteoroid has ever been found. The explosion was so vast—flattening and incinerating over an 800 square-mile swath of trees—that generations of amateur sleuths have put forward scenarios as strange as stray black holes or UFO attacks to explain the tremendous explosion.

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Now, a controversial scientific study suggests that a chunk of a comet caused the 5 to 10 megaton fireball—what amounts to the largest non-nuclear explosion in modern history. Crucially, according to the new hypothesis, most of the comet bounced off the atmosphere and back into orbit around the sun. The scientists have even identified a candidate Tunguska object—now more than 100 million miles away—that was somewhere near Earth on June 30, 1908 and will be passing close to Earth again in 2045. But just how could a comet—basically a ball of water ice and cosmic dust—create such a massive explosion and leave no trace? The answer, the scientists believe, can be found in basic chemistry rather than complicated physics or evidence yet to be found.

One of the most peculiar leftovers from the Tunguska event is the pattern of scorched earth it left behind. No mere circular blast, Tunguska's trail of charred trees fan out like a butterfly, with outer "wings" that spread both in a north-northeast direction and a south-southeast direction. When presented with this unusual pattern, top scientists, including Giuseppe Longo of the University of Bologna in Italy and Yuri Medvedev of the Russian Institute of Applied Astronomy, have circled around two main theories—that either two separate objects exploded in the skies over the region or one object skipped over the atmosphere, circled the earth and then re-entered over the Tunguska region on its second pass.

Edward Drobyshevski, a research physicist at the Russian Academy of Sciences in St. Petersburg, thinks instead that the comet fragment packed its own explosives—in the form of hydrogen, the gas that turned the Hindenburg airship into a blazing inferno in seconds.

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To come to his hydrogen theory, Drobyshevski focused on basic chemistry, namely electrolysis—the chemical splitting of water into its hydrogen and oxygen components using electricity. The hydrogen that caused the 1908 explosion, Drobyshevski says, most likely comes from the comet's earlier incarnation—as a tiny part of an ice sheet on a moon of Jupiter or Saturn. Over time, the strong magnetic fields from the host planet split some of the water molecules in the ice sheet into little hydrogen and oxygen bubbles that remained trapped within it. Once enough hydrogen had accumulated in the ice sheet, a direct asteroid hit would have caused the sheet to explode, sending shards of hydrogen-bubble-filled ice into space. (As far back as 1981, Drobyshevski published a paper theorizing that hydrogen-rich ice on Saturn's moon Titan exploded between 3000 and 10,000 years ago, enriching the structure of Saturn's rings and perhaps also sending some volatile icy shards into the solar system.)

These ice shards then wander the solar system, Drobyshevski says, and occasionally cross paths with a planet like Earth, as one may have done in 1908. Add a spark to one of these shards from, say, the friction of entering an atmosphere and you have one giant, naturally developed, explosive ice-bomb.

When the Tunguska meteoroid skimmed the Earth's atmosphere, according to Drobyshevski's calculations, 10 percent of its ice exploded. This then kicked the parent meteoroid back into space. Such a jagged trajectory would explain the unusual butterfly pattern of felled trees, he says. Hunting through databases of more than 6000 near-Earth objects, Drobyshevski and collaborators found one, called 2005NB56, that passed outside of the moon's orbit around June 27, 1908. The timing and the direction that 2005NB56 would have been coming from is a close fit with eyewitness accounts of the Tunguska event, according to Drobyshevsky. (The object's closest calculated distance from the Earth, 6.2 million miles, carry large error margins, he says.)

Comet 2005NB56 is also going to be passing within 3.8 million miles of the Earth in 2045, about 16 times the distance from the Earth to the moon. This time around, it may not pose an imminent threat to the planet, Drobyshevsky says, but studying the object as it approaches could yield important insights about near-Earth objects—and what unforeseen dangers may be stored in their ice.

A colleague from the Russian Academy of Science, Yuri Medvedev of the Institute of Applied Astronomy in St. Petersburg, is impressed but not convinced by Drobyshevski's argument that 2005NB56 caused the Tunguska explosion. Medvedev says that Drobyshevski's theory of comets' explosive origins—and a possible explosive encounter with Earth in 1908—are far from proven, but deserve more attention from astronomers.

"Drobyshevski's theory explains some unusual observations," Medvedev told PM via email. For instance, he says, when NASA's Deep Impact spacecraft dropped a probe onto the surface of the comet Tempel 1 in 2005, a small part of the comet exploded with unexpected fury—consistent with the hypothesis that some comets carry their own hydrogen pyrotechnics.

The research conducted by Drobyshevskia and his team has been submitted for publication to the journal Icarus.

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